Elderly, very young and immunocompromised individuals are particularly susceptible to infection by Streptococcus pneumoniae (S. pn.). We have previously shown that an immune response to phosphorylcholine (PC), an antigenic determinant expressed on the surface of S. pn. and many other pathogens is sufficient to confer protection to challenge by S. pn. Recently we have demonstrated that the mouse VH1 immunoglobulin heavy chain gene is necessary and sufficient to generate a PC-mediated protective immune response against S. pn. Subsequently we have defined structural determinants of immunoglobulin light chains which may explain the observed differences in the relative affinity/avidity of VH1/VL antibodies for PC and are continuing to examine the complimentary contribution of specific VH and VL genes to defining a protective versus an ineffective immune response. As stated, PC is expressed on the surface of many pathogens and this makes it an ideal antigen for vaccine development. We have recently developed and patented a novel, inexpensive strategy for synthesizing PC-containing compounds which can be used to generate PC-protein conjugates. Immunization of mice with PC-conjugates prepared via this novel synthesis strategy resulted in a long lasting increase in PC-specific antibody titer. In an attempt to develop PC-protein conjugates for use as a vaccine in humans, ongoing experiments are examining questions about the efficacy of various PC-conjugates, immunization regimens and combinations with and without adjuvants to generate a protective PC-specific immune response. Mouse models are being developed to evaluate the ability of PC-protein conjugates to provide protection against challenge by numerous other pathogens and studies have been initiated to define the PC-specific response in humans. [unreadable] [unreadable] [unreadable] Based on our observations in PC transgenic mouse models, we proposed that compromising a central tenet to the clonal selection theory (single B cell, single specificity, and single antigen receptor) was beneficial and necessary for the host. We suggested that in order to prevent the elimination of B cells expressing certain immunoglobulin receptors that were autoreactive but that had been evolutionarily selected to confer protection against various pathogens; these cells expressed two distinct receptors with different specificities. We termed this mechanism of rescuing autoreactive cells receptor dilution . In our current studies we have demonstrated the generality of this mechanism by showing that dual isotype expressing B cells are present in normal, nontransgenic C57BL/6 mice. These studies also indicated that dual isotype expressing B cells most likely arise during normal B cell development in the bone marrow and that a significant proportion of dual isotype expressing B cells may be generated by receptor editing. Consistent with our original hypothesis, the inferred specificities of the VH and VK genes expressed by this small population of B cells are both self antigens and antigens expressed on numerous pathogens. Ongoing studies continue to extend the receptor dilution model in order to provide a greater understanding about the role of dual receptor expression in establishing and maintaining a protective immune repertoire. [unreadable] [unreadable] [unreadable] We have demonstrated that TdT (terminal deoxynucleotidyl transferase) plays an important role in shaping the PC-specific immune repertoire. Our studies established that generation of the dominant M603id (idiotype) anti-PC response to immunization with Proteus morganii is dependent on TdT expression. The impact of TdT expression on shaping the immune repertoire has been extended in subsequent studies in which we have revealed that TdT expression affects the size of the autoreactive, PtC (phosphatidylcholine)-specific B1 B cell population and most likely the VH and VL gene repertoire expressed in these PtC-specific B cells. We continue to examine B cell populations in the TdT-KO mice to determine the identity of the VH and VL genes expressed in this expanded autoreactive PtC-specific population and to determine the mechanism by which TdT shapes the size of and VH/VL gene repertoire of this and other autoreactive B cell populations.